The present invention relates to a high resolution full color exposure device using masks electronically generated, i.e., from computer output, as color separations.
As those skilled in the art know, color separations are masks which are used in the exposure of photosensitive materials useful in generating a full color image. Each color separation whether blue, green, or red is generated individually. Blue, green, and red are the additive primary colors; their subtractive couples are yellow, magenta, and cyan respectively. Typically, photosensitive media produce full color images from combinations of the subtractive primaries-yellow, magenta, and cyan. In color separations, exposure of media through transparent areas of the blue separation exposes out (controls) yellow so that it does not develop in the image in exposed areas; similarly, the green separation controls magenta, and the red separation controls cyan. The separations are called "blue," "green," or "red" and not "yellow controlling," "magenta controlling," and "cyan controlling" due to a convention arising from two facts . First, control of yellow yields blue but only if there are no subsequent re-exposures for controlling magenta and/or cyan in these areas; similarly, control of magenta yields green and control of cyan yields red. Secondly, in a panchromatic system such as conventional photography, blue light controls yellow through the blue separation; green light controls magenta through the green separation; red light controls cyan through the red separation. Typically, after a set of separations is produced, a photosensitive material is exposed to a source of actinic radiation of the appropriate wavelength through each of the three separations such that the desired colors form upon subsequent development. Thus, after three exposures, subsequent development of the photosensitive material yields a full color image.
The blue, green, and red exposure wavelengths may be either matched to these colors as in a panchromatic system or may be any other set of three wavelengths such as ultraviolet or a combination of ultraviolet, visible, or even infrared. Except for laser light which is monochromatic (one wavelength), referral to wavelength also implies a bandpass. Bandpass is defined as the range of wavelengths centered on the given wavelength which are used in the exposure (i.e., for a wavelength of 580 nm and bandpass of 20 nm, the wavelengths included are 570 to 590 nm).
Proposals to use liquid crystal displays (LCD) or piezoelectric light transducers (PZLT) as electronically controlled light modulators for exposure of photosensitive material have several inherent problems. Both suffer from limited resolution capabilities and significant loss of light through the devices. Where exposure of the material is done by scanning one row of dots (pixels) or one dot at a time, dwell time of the light on the material can be on the order of thousandths or millionths of a second for reasonable image production times. It, therefore, becomes necessary to transmit very high intensities of light through the device onto the material. Due to the light loss previously mentioned, extremely high intensity light sources are necessary. Such light sources are expensive and produce significant amounts of heat. Another problem with using high intensity light is high intensity reciprocity failure observed in both conventional photographic materials and the microencapsulated system described later in this document. High intensity reciprocity failure results in failure to control the dye in the exposed areas, appearing to be a reversal in extreme cases.
A method and apparatus for magnetic-optical printing employing a light-transparent uniformly magnetized magnetic medium is disclosed in U.S. Pat. No. 3,604,324. A magnetic substrate is uniformly magnetized and then image-wise demagnetized using a magnetic write-in means. The substrate is dusted with a magnetic toner which selectively attaches to the substrate to form a mask image through which a photographic material may be exposed.
Transparent photoconductors are commercially available. The photoconductors are first uniformly charged and then exposed to light to eliminate the charge in the exposed areas and form a latent image on the surface. Toner particles are then deposited in the charged areas.
A method for photoimaging a photosensitive layer is disclosed in U.S. Pat. No. 4,429,027. In a disclosed drum transfer technique, a drum transfer roll having a toner image on its surface transfers the toner on contact to a cover sheet which has been heated sufficiently to accept the toner. A photosensitive material is subsequently exposed through the applied toner on the cover sheet and the cover sheet is removed.